A change-speed transmission that can be shifted manually by means of a hand-operated shift lever or automatically by means of at least one auxiliary-force-actuated gear control element usually comprises a plurality of gearsets with different transmission ratios, which are arranged between two axis-parallel transmission shafts and in each case have a fixed wheel attached in a rotationally fixed manner on one of the transmission shafts and a loose wheel mounted to rotate on the other transmission shaft. To engage a gear, the loose wheel of the gearset concerned is connected rotationally fixed to the transmission shaft concerned by engaging an associated gear clutch. The gear clutches can be unsynchronized, i.e. in the form of so-termed claw clutches, or friction-synchronized, i.e. as so-termed synchronous clutches. The gear clutches are usually combined in pairs in a common shifting packet, in each case with a shift sleeve arranged in a rotationally fixed manner but able to move axially on the transmission shaft concerned. Consequently, the engagement and disengagement of the gears associated with a shifting packet takes place by axial displacement of the shift sleeve in respectively opposite directions.
The transmission-internal portion of the shifting device for actuating the shift sleeve can comprise several shift rods, which are arranged axis-parallel to the transmission shafts and are mounted to move axially within the transmission housing, and to which in each case is solidly attached a shift rocker which engages with an associated shift sleeve. To engage a gear, the shift rod concerned is mechanically coupled in a selection process using the manual shift lever or gear control element, and is displaced axially in the subsequent shifting process. The disadvantages of a shifting device of this type, however, are the relatively large structural space they occupy for the arrangement and mounting of the shift rods, and the comparatively large shifting forces needed to engage and disengage the gears.
In another form of the shifting device which takes up less space and requires lower shifting forces, the transmission-internal portion comprises a plurality of shift rockers each having a basic U-shape and consisting of two rocker arms connected to one another by a rocker bridge, which are arranged over a shifting packet of a transmission shaft that comprises a shift sleeve, which engage with an annular groove of the shift sleeve concerned by means of two slide-blocks respectively attached at the end of each rocker arm on a holder, which are mounted to pivot about a transverse axis determined by two bearing points located respectively on each of the rocker arms, and which have a carrier element arranged on the V bridge to engage with a shifting pin of a shifting shaft arranged axis-parallel to the transmission shaft. The shifting shaft is mounted to rotate and move axially within the transmission housing and comprises a plurality of shifting pins. In a selection process the shifting shaft is rotated about its longitudinal axis and is coupled to a shift rocker when one of the shifting pins engages with the carrier element of the rocker, and in the subsequent shifting process it is displaced axially whereby the shift rocker is pivoted about its transverse axis.
For the selection process, i.e. for coupling the shifting shaft in each case to one of several shift rockers, in order to ensure a sufficient angular resolution and a secure engagement of the shifting pin in the respective carrier element over the entire travel path for the shifting operation, the shifting shaft is usually arranged centrally above the shift rockers but the carrier elements are correspondingly fixed off-center on the rocker bridges of the shift rockers. However, this has the disadvantage that the shifting force is applied asymmetrically to the shift rocker and the shifting force is transferred unequally, via the rocker arms of the shift rockers, to the respective shift sleeve.
Since with the usual, symmetrical design of the shift rocker arms the shorter part of the shift rocker formed by the rocker arm closer to the carrier element and the adjacent, shorter section of the rocker bridge has less spring elasticity because of its smaller overall length that the longer part of the shift rocker formed by the rocker arm farther away from the carrier element and the adjacent, longer section of the rocker bridge, the shifting force is transferred to the shift sleeve for the most part by the more rigid, shorter part of the shift rocker and correspondingly on one side of the sleeve. Besides this unfavorable force transfer, the intensive contact of the shift sleeve with the more rigid, shorter part of the shift rocker also favors an undesired transmission of oscillations and vibrations from the transmission to the manual shift lever or gear control element of the shifting device.
A typical shift rocker with a symmetrical rocker arm design is described for example in DE 199 19 270 A1. The rocker bridge and the two rocker arms of this known shift rocker are made as an integral pressed and stamped sheet component. The rocker arms are made as simple brackets and are each provided at the end with a fitting bore for the attachment of a slide-block and, approximately centrally, with a bearing bore for the engagement of a bearing bolt of a pivot bearing. On the rocker bridge a rail that projects on one side is fixed by means of a substantially central weld, and on this rail among other things a fork-shaped carrier element for engaging a shifting pin of a shifting shaft is attached off-center. The central welding of the rail to the rocker bridge is designed to ensure symmetrical application. However, owing to the effective lever arm between the carrier element and the rail weld, there is a tilting torque which results in asymmetrical distribution and transfer of a shifting force in or via the two rocker arms.
Another embodiment of a shift rocker with symmetrically designed rocker arms is known from DE 10 2005 034 283 A1. The shift rocker illustrated in FIG. 4 of that document and explained in the associated description comprises a carrier element arranged off-center for engaging a shifting pin of a shifting shaft. The rocker arms are made identically to one another and have at the end in each case a U-shaped curve extending in the pivoting direction, at the curve bottoms of which are respective fitting bores for the attachment of a slide-block and at the free ends of which there is in each case a bearing aperture for engaging a bearing bolt. Due to their curved ends the rocker arms have lower elastic rigidity compared with rocker arms made as brackets, whereby the transfer of oscillations and vibrations from the transmission to other parts of the shifting device can be damped. However, the asymmetrical distribution and transfer of a shifting force from the carrier element in or via the two rocker arms is still a disadvantage.
To avoid that disadvantage a shift rocker is proposed in EP 1 472 478 B1 (DE 603 01 489 T2), in which the rocker arm closer to the carrier element arranged off-center has a lower elastic rigidity than the other rocker arm farther away from the carrier element. Owing to this lower elastic rigidity of the rocker arm nearer the carrier element, achieved by making it with a smaller cross-sectional area, during the transfer of a shifting force it yields elastically to the extent that a substantial fraction of the shifting force is also transferred by the other rocker arm. In the example embodiment described, the rocker arms are made largely identically and have at their end in each case a U-shaped curve extending in the pivoting direction, with an inner leg attached to the rocker bridge and an outer leg connected to this via a curve bottom. At the free end of the outer leg there is in each case a fitting bore for fixing a slide-block. A bearing bore for engaging a bearing bolt of a pivot bearing is provided at the bottom of the curve or in the middle of the inner leg. The smaller cross-sectional area intended to reduce the elastic rigidity of the rocker arm closer to the carrier element is in this case produced by reducing the material thickness of the inner leg concerned. But since the elasticity of both rocker arms is determined essentially by the considerably smaller material thickness of the outer leg, the efficacy of this known design is questionable. Besides, reducing the cross-section of one of the rocker arms can reduce the fatigue strength of the shift rocker.